1. Field of the Invention
The present invention relates to the calibration of transmitter/receivers. More particularly, it relates to QPSK transmitter/receiver IC's and a method and apparatus for the self-calibration and testing thereof.
When communicating with cellular phones, both sides of the communication are transmitting and receiving signals. Upon transmission, the voice signal is modulated and transmitted. The receiver receives the signal and demodulates it to reproduce the transmitted signal. Ideally, the resulting demodulated signal should be identical to the transmitted signal. However, variable parameters such as gain mismatch, offset voltages and phase errors inherent in the signal and circuit cause the demodulated signal to vary from the transmitted signal.
Generally with GSM cellular phones, a Quadrature Phase Shifted Keying (QPSK) modulation scheme is used. QPSK has four vector components for the signals, each of which are 90.degree. apart. This differs from a PSK modulation scheme where there are two vector components 180.degree. apart (e.g., one at 0.degree. and the other at 180.degree.). With QPSK, we have vector components at 45.degree. with a coordinate (1,1); 135.degree. with a coordinate (-1,1); 225.degree. with a coordinate (-1,-1); and, 315.degree. with a coordinate at (1,-1) where the X-axis is the Q channel (quadrature phase) and the Y-axis is the I channel (in phase). For purposes of calculation of the variable parameters, the Q channel can be designated Cosine, and the I channel can be designated Sine.
The amplitude of the four vector components for QPSK, under ideal conditions, would be .sqroot.2. However, since the circuits do not operate ideally, when one modulates a signal and sends it, the demodulated signal on the receiver end does not have the same vector coordinates and amplitude as the originally transmitted signal. Thus, the received signal does not contain the same data.
These inaccuracies are caused by errors in both the transmit and receive channels and mixers. Gain mismatch and offset voltages in both the transmit and receive mixers, in addition to phase errors inherent in both channels result in vector positions that are different than those transmitted.
It would therefore be desirable to have a circuit that enables the performance of a self-calibration technique for determining these error components and which subsequently adds them back to the system in order to numerically correct the demodulated output of the receiver mixer.